1. Field of the Invention
This invention pertains generally to the synthesis of copolymers of 3-alkoxythiophene, and more particularly to the use of regioregular copolymers of 3-alkoxythiophene in photovoltaic applications.
2. Description of Related Art
Conjugated polymers have been developed into useful materials for a variety of applications, including light-emitting diodes, photovoltaic cells (PVs),3-5 and thin-film transistors (TFTs). In the past few years, photovoltaic devices based on conjugated polymers have been extensively studied. The most widely used configuration of polymer solar cells is the so-called “bulk heterojunction” devices in which the active layer consists of a blend of an electron-donating materials, e.g., a p-type conjugated polymer, and an electron-accepting (n-type) material such as (6,6)-phenyl C61-butyric acid methyl ester (PCBM). Photo-induced charge transfer from a conjugated polymer to PCBM with quantum yields up to 100% has been obtained. Regioregular poly(3-alkylthiophene)s (P3ATs) have been found to be one of the most promising conjugated polymers. They can be used as photosensitizers and hole transporters in bulk heterojunction polymer solar cells. Power conversion efficiencies (PCE) exceeding 3% under AM1.5 G illumination and between 4 and 5% under white light illumination from a solar simulator have recently been reported. Further improvement on the PCE entails new conjugated polymers with higher carrier mobility and broader absorption of the solar spectrum, especially in the red and infrared range. Moreover, the relatively low PCE of the polymer cells is largely due to low open-circuit voltage (Voc). The maximum open-circuit voltage is limited by the difference between the electronegativity, i.e., the lowest unoccupied molecular orbital (LUMO) of PCBM and the polymer's ionization potential, i.e., the highest occupied molecular orbital (HOMO). Therefore, HOMO level is also an important parameter to consider when designing new, electron-donating polymers of low bandgap.
Polythiophenes with substituents other than alkyl groups have also been investigated, among which those with electron-donating alkoxy groups have displayed promising electronic and optical properties. Compared to P3ATs, the incorporation of an alkoxy group to the 3-position of the thiophene ring yields poly(3-alkoxythiophenes) (P3AOTs) with optical absorption maxima at longer wavelength. This may be attributed to both the electron-donating effect of alkoxy group and the more coplanar conformation of the P3AOTs. Therefore, polymers and copolymers based on 3-alkoxythiophene may also have smaller bandgaps than those based on P3ATs, and they can more efficiently absorb the red and near infrared portion of the solar emission spectrum.
Efforts to synthesize new conjugated polymers for photovoltaic application have been undertaken, beginning with regioregular poly(3-decyloxythiophene-2,5-diyl) (P3DOT), but it was found that thin films of P3DOT did not have sufficiently high uniformity and environmental stability.